Mass spectrometry has proven to be an effective analytical technique for identifying unknown compounds and determining the precise mass of known compounds. Advantageously, compounds can be detected or analyzed in minute quantities allowing compounds to be identified at very low concentrations in chemically complex mixtures. Not surprisingly, mass spectrometry has found practical application in medicine, pharmacology, food sciences, semi-conductor manufacturing, environmental sciences, security, and many other fields.
A typical mass spectrometer includes an ion source that ionizes particles of interest. The ions are passed to an analyser region, where they are separated according to their mass (m)-to-charge (z) ratios (m/z). The separated ions are detected at a detector. A signal from the detector is provided to a computing or similar device where the m/z ratios are stored together with their relative abundance for presentation in the format of a m/z spectrum.
Typical ion sources are exemplified in “Ionization Methods in Organic Mass Spectrometry”, Alison E. Ashcroft, The Royal Society of Chemistry, UK, 1997; and the references cited therein. Conventional ion sources may create ions by atmospheric pressure chemical ionisation (APCI); chemical ionisation (CI); electron impact (EI); electrospray ionisation (ESI); fast atom bombardment (FAB); field desorption/field ionisation (FD/FI); matrix assisted laser desorption ionisation (MALDI); or thermospray ionization (TSP).
Ionized particles may be separated by quadrupoles, time-of-flight (TOF) analysers, magnetic sectors, Fourier transform and ion traps.
The ability to analyse minute quantities requires high sensitivity. High sensitivity is obtained by high transmission of analyte ions in the mass spectrometer, and low transmission of non-analyte ions and particles, known as chemical background.
Many known mass spectrometers produce ionized particles at high pressure, and require multiple stages of pumping with multiple pressure regions in order to reduce the pressure of the analyser region in a cost-effective manner. Vacuum pumps and multiple pumping stages reduce the pressure in a cost-effective way, decreasing the gas load along various pressure stages.
Because most useful ion sources operate at high pressure, and most useful mass spectrometers operate at lower pressure, ions must be transported from regions of higher pressure to lower pressure. Conventionally, an associated ion guide transports ions through these various pressure regions. An ion guide guides ionized particles between the ion source and the analyser/detector. The primary role of the ion guide is to transport the ions toward the low pressure analyser region of the spectrometer. For high sensitivity low ion losses at each stage are desirable.
At the same time, the sensitivity of the mass spectrometer depends at least in part on the inlet orifice from atmosphere. However larger orifice diameters put more gas load on the system. Often the ion guide includes several such stages of accepting and emitting the ions, as the beam is transported through various vacuum regions and into the analyser. Conventional mass spectrometers utilize large differential pressure drops from stage to stage, for example typically 100-1000 fold, in order to remove the gas load quickly, in an attempt to focus the ion beam in an ion guide.
Unfortunately this approach causes a reduction in sensitivity due to scattering losses that occur at the transition points from stage to stage. For example, as the ion and gas exit a high pressure region into a lower pressure region, the ion beam may be entrained in a flow of high density gas. The ions in the high density gas cannot be readily guided or concentrated. Ions may be scattered in the high density gas, and lost to the surroundings.
Accordingly, there is a need for an improved mass spectrometer, including multiple pressure stages that may provide for smoother transport of ions from a high pressure region to a lower pressure region.